How Is Steel Made? Iron, Carbon and the Furnace
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Iron straight from the furnace is brittle, and pure iron is soft; steel is what lies between, iron with somewhere between a tenth and one percent of carbon dissolved in it, and that narrow band is the material of every bridge, car, ship, rail, skyscraper and kitchen knife. About 1.9 billion tonnes of it are made each year, more than all other metals combined by a factor of twenty, and the making of it accounts for about eight percent of the world's carbon dioxide. The chemistry is two thousand years old and the industrial process a hundred and seventy, and the next change to it is under way.
From ore to iron
Iron in the ground is rust, oxides of iron in ores such as haematite and magnetite, and getting the metal out means removing the oxygen. The blast furnace does that with carbon. Ore, coke, which is coal baked to nearly pure carbon, and limestone are fed in at the top of a tower thirty metres high, and hot air is blasted in at the bottom. The coke burns to carbon monoxide, the carbon monoxide strips the oxygen from the ore as the charge descends, and molten iron collects at the bottom with a layer of slag, the limestone having gathered up the ore's rocky impurities, floating on top. The furnace runs without stopping for ten or fifteen years. What comes out is pig iron, about four percent carbon, with silicon, manganese, phosphorus and sulphur, and it is hard, brittle and useless for anything but casting.
From iron to steel
Steelmaking is the removal of most of that carbon and the impurities, and the modern method is to blow oxygen through the molten iron. In the basic oxygen furnace, a pear-shaped vessel holding up to 400 tonnes, a water-cooled lance lowered into the melt blows pure oxygen at supersonic speed for about twenty minutes; the carbon burns off as carbon monoxide, the silicon and manganese oxidise into the slag, and the heat of the reactions keeps the metal molten with no fuel at all, so that scrap steel is added to cool it. The process, developed in Austria in 1952, does in twenty minutes what earlier furnaces took hours over, and it makes about seventy percent of the world's steel. The other thirty percent comes from electric arc furnaces, which melt scrap steel with an electric arc and are the main route for recycling; steel is the most recycled material on Earth, and a car's body is likely to have been a car before.
Why the carbon matters
Iron atoms pack in a crystal lattice, and the layers of the lattice slide past each other easily, which is why pure iron is soft. Carbon atoms, much smaller, sit in the gaps between iron atoms and jam the layers, so a little carbon makes the metal harder and stronger; too much and it forms brittle iron carbide throughout and the metal cracks instead of bending. Heat treatment adjusts the result. Steel heated red and quenched in water traps the carbon in a distorted lattice called martensite, glass-hard and brittle, and reheating it gently, tempering, trades some hardness for toughness, which is how a sword or a chisel gets a hard edge on a body that will not shatter. Alloying elements add the rest:
- •Chromium, above about 11 percent, forms a self-healing oxide skin and makes stainless steel
- •Nickel keeps stainless steel tough at low temperatures and is why cutlery is 18/8, eighteen percent chromium and eight nickel
- •Manganese, in nearly all steel, ties up sulphur and adds strength; at 12 percent it makes the steel of railway points and rock crushers, which hardens where it is hit
- •Tungsten, molybdenum and vanadium make tool steel that stays hard red-hot
- •Silicon in transformer steel, boron in armour, and a few hundredths of a percent of niobium in the high-strength steel of pipelines and car bodies
From Bessemer to now
Steel was made in small quantities for two thousand years, in Damascus, in Japan and in the crucibles of Sheffield, by craftsmen who could not have said what carbon was. The industrial age began in 1856 when Henry Bessemer blew air through molten pig iron in a converter and found the carbon burned out in minutes, turning a material that had cost as much as silver into one that could be made by the hundred tonnes; the open-hearth furnace of the 1860s refined the method, and the price of steel fell by four fifths within thirty years. Rails, ships, the Eiffel Tower's rival in Chicago, the first skyscrapers and the armoured warships of the naval race all followed, and the oxygen furnace and continuous casting after 1950 finished the job. China now makes about half the world's steel.
The carbon problem
Every tonne of steel from a blast furnace releases about 1.8 tonnes of carbon dioxide, most of it from the coke that reduces the ore, and the process has no alternative reducing agent in commercial use. Two are coming. Hydrogen can strip the oxygen from iron ore as carbon monoxide does, producing water instead of carbon dioxide, and the first hydrogen-reduced steel was delivered in Sweden in 2021, with commercial plants under construction. And electrolysis, which extracts aluminium from its ore with electricity, is being adapted for iron. Both need very large amounts of clean electricity, which is the constraint; until it is lifted, the world's most useful material remains one of its largest sources of the gas that is warming it.
The takeaway
Steel is iron with a small, controlled amount of carbon, made by first reducing iron ore with coke in a blast furnace to brittle pig iron and then burning most of the carbon out with a blast of oxygen, or by melting scrap in an electric arc furnace. The carbon atoms lodged between the iron atoms give the metal its strength, heat treatment and alloying tune it for every use, and the coke that does the reducing is why steelmaking produces eight percent of the world's carbon dioxide, a problem hydrogen is beginning to address.